Euphausiid Community Research Articles

Primary productivity impacts community structure of euphausiids in the low-latitude Indian and Pacific Oceans

Euphausiids are a vital component of global marine micronekton. To reveal the primary environmental factors influencing euphausiid distribution patterns in the previously overlooked low-latitude ecosystems, we investigated a large-scale community structure of euphausiids covering the North Pacific subtropical gyre (NPSG) and low-latitude eastern Indian (EI), and South Pacific Oceans (SP). A total of 41 euphausiid species from six genera were identified. Integrated primary production (PP) correlated significantly with the euphausiid abundance and species diversity and displayed the most critical influence on the variations in euphausiid community structure in low latitudes. Dissolved oxygen (DO) was the second significant environmental driver. Due to the distinct distribution patterns of euphausiid species in response to different PP and DO levels, the low-latitude euphausiid assemblages were mainly distinguished into subtropical and tropical communities. The subtropical euphausiid community associated with lower PP and higher DO demonstrated significantly lower euphausiid abundance but higher diversity than the tropical community. Euphausia brevis, E. mutica, and Stylocheiron abbreviatum, which characterize the subtropical community, appear to depend less on the phytoplanktonic prey. Euphausia diomedeae and Hansarsia gracilis, typifying the tropical population, showed stronger hypoxia tolerance. Additionally, each community was further divided into three subgroups under the influence of surface PP (subtropical: marginal NPSG, central NPSG, and austral EI-SP subgroups; tropical: the Bay of Bengal, equatorial EI, and equatorial EI-SP subgroups). These results suggested that food resource is the most important in shaping euphausiids’ community structure in an oligotrophic ecosystem with subtle hydrography gradients.

Interannual response of euphausiid community abundance during the anomalous warming period (2014–2016) in the Gulf of California

El Niño in the northeast Pacific and Gulf of California typically causes a decrease in euphausiid abundance (biomass), and modifies their latitudinal biogeographic distributions, but the effect of recent marine heat waves (or their combined effect with El Niño) are less understood. The interannual response of euphausiid community structure and abundance were investigated during the cold–warm transition period (June) of six years (2013–2019, except 2015) in the central Gulf of California, Mexico. Abundance in June 2013–2019 was compared with historical baseline for euphausiid abundance made in June 1957 from a California Cooperative Oceanic Fisheries Investigation cruise, just before El Niño 1957–1958. During 2014–2019 two anomalously warm events occurred: the marine heat wave of 2014 (MHW 2014) and El Niño 2015–2016 (May 2015–May 2016). We tested the hypothesis that the numerically dominant subtropical neritic species Nyctiphanes simplex and temperate oceanic Nematoscelis difficilis significantly decrease in abundance in response to the prolonged warming that occurred during 2014–2016 and that tropical euphausiid species could increase in relative abundance during this period. Ten euphausiid species were collected accounting for < 3% of total relative abundance of 34 zooplankton taxonomic groups collected during June 2013–2014, 2016, 2018 and between 7 and 22.4% during 2017 and 2019. N. simplex and Ne. difficilis accounted for > 80% of euphausiid total abundance during June 2013–2019, with considerably lower abundance during 2013, the MHW 2014, and El Niño 2015–2016 than in June 2017, 2018, and 2019. Mean euphausiid abundance in 2017 and 2019 was slightly higher than that recorded in June 1957. Tropical species increased their relative abundances during El Niño 2015–2016 event, but they were always < 15% of total euphausiid relative abundance during 2013–2019. We estimated that it took at least 3 years for the euphausiid species abundance in the gulf to slowly recover from the unfavorable conditions associated with MHW 2014, and El Niño 2015–2016 to attain highest mean abundance in 2019. Multi–variated ordination analyses showed that the prolonged warming and anomalously low Chl-a concentrations recorded in 2014–2016 negatively influenced euphausiid abundance, a proxy for euphausiid biomass available for pelagic predators in the Gulf of California.

Feeding Ecology of Three Euphausiid Species in the North Pacific Ocean Inferred From 18S V9 Metabarcoding and Stable Isotope Analysis

Euphausiids are abundant micronekton and important links between higher and lower trophic levels in marine ecosystems; however, their detailed diets cannot be fully understood by conventional microscopy, especially in subtropical areas. Here, we report the euphausiid community structure in the California Current (CC) area and the eastern/western North Pacific subtropical gyre (ESG and WSG) and detail the feeding ecology of the dominant species (Euphausia pacifica, E. brevis, and E. hemigibba) in each region using a combined approach of gut content analysis via 18S V9 metabarcoding and stable carbon and nitrogen isotope analysis. A pronounced omnivorous feeding of all studied euphausiid species was supported by both methods: phytoplanktonic taxonomic groups (Dinophyta, Stramenopiles, and Archaeplastida), Copepoda, and Hydrozoa were detected in the gut contents; all the three euphausiid species displayed an intermediate trophic position between the net plankton (0.2–1.0 mm) and the myctophid fish (15.2–85.5 mm). However, Hydrozoa found in euphausiid gut contents likely derived from a potential cod-end feeding, based on isotope analysis. E. pacifica in the CC province ingested more autotrophic prey, including pelagophyte and green algae, due to a greater abundance of Stramenopiles and Archaeplastida in shallow layers of CC water. On the other hand, non-autotrophic prey such as mixotrophic Kareniaceae dinoflagellates, Pontellidae and Clausocalanidae copepods, and Sphaerozoidae rhizarian contributed more to the diets of E. brevis and E. hemigibba because of a lower chlorophyll a concentration or potentially a scarcity of autotrophic prey availability in ESG and WSG. The feeding patterns of dominant euphausiid species conducting filter feeding were thus largely determined by phytoplankton prey availability in the environments. Dietary difference across three species was also indicated by stable isotope analysis, with a lower mean trophic level of E. pacifica (2.32) than E. brevis (2.48) and E. hemigibba (2.57). These results verify direct trophic interactions between euphausiids and primary production and suggest that the omnivorous feeding habit is a favorable character for dominant Euphausia species.

Unusually low euphausiid biomass during the warm years of 2014–2016 in the transition zone of the California Current

Euphausiids represent a significant part of the total zooplankton biomass and are a key link in the trophic web of the California Current System (CCS). During 2014–2016, the northeast Pacific experienced warming caused by two successive events, the Blob and the 2015–2016 El Niño, which affected the whole ecosystem. Here we present evidence for a strong impact of the 2014–2016 warm events on euphausiids in the Baja California region, namely two species (Euphausia pacifica and Nematoscelis difficilis) that are dominant in the CCS transition zone. Both species decreased approximately 95% in relation to the previous years (1999–2010). A similar decrease was observed for E. pacifica during the 1997–1998 El Niño, but the decrease in N. difficilis was higher in 2014–2016 and unprecedented during the 1997–2016 period. Biomass of the coastal species Nyctiphanes simplex showed negative anomalies during the Blob but not during the 2015–2016 El Niño. The tropical species Euphausia eximia and Euphausia recurva increased in biomass during the Blob in 2014 relative to the previous years, and even more during the 2015–2016 El Niño. The strong increase of E. eximia, E. recurva, and N. simplex during El Niño may have been due to upwelling activity in winter 2015–2016. Canonical correspondence analysis (CCA) was performed to assess the relation between the euphausiid community and environmental variables. The most influential variables were temperature and chlorophyll, and to a lesser extent salinity. The subarctic E. pacifica showed covariance with chlorophyll while a set of tropical species did with thermohaline conditions. However, the variance accounted for was low (20% taking the first three axis). N. difficilis did not show significant scores in the CCA suggesting that other variables such as availability of prey could be important for this carnivore species.

Specific Composition and Distribution of Euphausiids in the Zone of the North Atlantic Subtropical Convergence

The euphausiid community was studied between the surface and upper bathypelagics northeast of the Azores Islands (46°–37° N, 14°–29° W) during the hydrological summer (August to September) of 1984 using a midwater trawl. In total, 11 species of euphausiids were identified. The two most common species, Meganyctiphanes norvegica and Thysanopoda acutifrons, were found in 43.8 and 40.8% of the samples, respectively. In terms of biomass, M. norvegica dominated the catches, its densities reaching up to 160 kg per trawling hour. The region is dominated by North Atlantic arctic–boreal and boreal species, followed by pan-oceanic meso- and bathypelagic, as well as tropical–subtropical and widespread tropical euphausiid species. The study area, which is situated in the zone of the Atlantic subtropical convergence, is subjected to seasonal migrations of the Subpolar Frontal system. Owing to this, a quasi-stationary ecotone is established, where arctic–boreal species are spatially replaced first by tropical–subtropical and then by widespread tropical elements. There are grounds to believe that the quasi-stationary ecotone lasts both for many years and from season to season. In the latter case, seasonal succession of species takes place, but the ecological structure of the ecotone remains almost constant.

Vertical pelagic habitat of euphausiid species assemblages in the Gulf of California

We describe the seasonal changes in the horizontal and vertical distribution and abundance of euphausiid species associated with seven physical and 61 biological variables in the Gulf of California (24–31°N). Euphausiid community structure was explored in the epipelagic habitat (<200m) in January, July, and October 2007 and in epipelagic to bathypelagic habitats (<1400m depth) in May 2015. Twelve euphausiid species comprising two distinct regional assemblages were identified. Nyctiphanes simplex and Nematoscelis difficilis were the most abundant species (>90%) in all cruises carried out in the 26–31°N region and Euphausia distinguenda in the 24–26°N region (mostly in October >90%). We confirmed that Euphausia gibboides and Nematobrachion flexipes inhabit the mesopelagic habitat, adapted to <1ml O2 l–1 environmental condition. Although Euphausia lamelligera and N. simplex populations were concentrated in well-oxygenated water (>3ml O2 l–1) near the surface (<50m), they were also detected in low densities in the hypoxic mesopelagic habitat (250–800m), but only at oxygen levels less than 0.09ml O2 l–1. Stylocheiron affine and Stylocheiron carinatum were numerous well below the seasonal thermocline (100–350m). Nematoscelis difficilis and N. simplex extended into the hypoxic mesopelagic habitat, but at low densities. Multivariate analyses showed six seasonal and regional krill assemblages, which are characterized by different species (Similarity percentage analysis). Canonical Correspondence Analysis show that, of 68 variables, three abiotic factors (temperature, dissolved oxygen, and OMZ depth) and eight biotic factors (accessory pigments; zeaxanthin and 19'-hexanoyloxyfucoxanthin, and the abundance of six copepod species; Candacia pectinata, Canthocalanus pauper, Centropages furcatus, Rhincalanus nasutus, Scolecithrix danae, and Temora discaudata), were the most influential variables associated with the vertical distribution and abundance of euphausiids. Krill‒copepod (predator–prey) relationships may be important determinants of daily and seasonal vertical distribution patterns. We conclude that the euphausiid vertical habitat reaches down to 900m depth (with historical records of six species collected between 1000 and 2280m) but with lower diversity and abundance in the mesopelagic and bathypelagic habitats than in the epipelagic habitat.

Climatic and ecological drivers of euphausiid community structure vary spatially in the Barents Sea: relationships from a long time series (1952–2009)

Euphausiids play an important role in transferring energy from ephemeral primary producers to fish, seabirds, and marine mammals in the Barents Sea ecosystem. Climatic impacts have been suggested to occur at all levels of the Barents Sea food-web, but adequate exploration of these phenomena on ecologically relevant spatial scales has not been integrated sufficiently. We used a time-series of euphausiid abundance data spanning 58 years, one of the longest biological time-series in the Arctic, to explore qualitative and quantitative relationships among climate, euphausiids, and their predators, and how these parameters vary spatially in the Barents Sea. We detected four main hydrographic regions, each with distinct patterns of interannual variability in euphausiid abundance and community structure. Assemblages varied primarily in the relative abundance of Thysanoessa inermis versus T. raschii, or T. inermis versus T. longicaudata and Meganyctiphanes norvegica. Climate proxies and the abundance of capelin or cod explained 30-60% of the variability in euphausiid abundance in each region. Climate also influenced patterns of variability in euphausiid community structure, but correlations were generally weaker. Advection of boreal euphausiid taxa from the Norwegian Sea is clearly more prominent in warmer years than in colder years, and interacts with seasonal fish migrations to help explain spatial differences in primary drivers of euphausiid community structure. Non-linear effects of predators were common, and must be considered more carefully if a mechanistic understanding of the ecosystem is to be achieved. Quantitative relationships among euphausiid abundance, climate proxies, and predator stock-sizes derived from these time series are valuable for ecological models being used to predict impacts of climate change on the Barents Sea ecosystem, and how the system should be managed.

Congruence between euphausiid community and water region in the northwestern Pacific: particularly in the Oyashio–Kuroshio Mixed Water Region

The relationship between euphausiid community structure and water region was studied during a 2-year seasonal survey in the northwestern (NW) Pacific Ocean. The euphausiid community structure and its associated species were analyzed from 38 micronekton samples collected during eight cruises. The euphausiid community structure and its distribution patterns clearly corresponded to physical oceanographic features in the Oyashio region, Oyashio–Kuroshio Mixed Water Region (OKMWR), and Kuroshio region. In contrast, community structure was unrelated to seasonality. The 19 species out of 40 identified in this area were grouped and named after their habitats. The six cold-water species were grouped into three regional types: two coastal Oyashio species, three Oyashio–OKMWR species, and one Oyashio–Kuroshio species. The four species dominating in the OKMWR were categorized into each specific types: Nematoscelis difficilis as OKMWR–Oyashio species, Euphausia gibboides as OKMWR species, Euphausia similis as OKMWR–Oyashio & OKMWR–Kuroshio species, and Euphausia recurva as OKMWR–Kuroshio species. The seven warm-water species were categorized as Kuroshio–OKMWR species or Kuroshio species. The other two species were categorized as cosmopolitan species. In particular, regarding the result in the OKMWR, our study suggest that (1) the OKMWR has high species diversity, and (2) the dominant species, such as Euphausia pacifica, N. difficilis, E. similis, and E. gibboides, are considered to be key species in the food webs in this region.

The influence of upwelling variation on the spatially-structured euphausiid community off central-southern Chile in 2007–2008

This study assesses the spatial and temporal patterns of the euphausiid community from the coastal upwelling zone and oceanic areas of central-southern Chile, surveyed between August 2007 and November 2008. A total of 22 species were found, most of them already described for the region. The numerically dominant species was Euphausia mucronata for the entire study area (>60%), which was found in more oceanic waters (>90°W), thereby expanding its longitudinal distribution range. The species showed distinct spatial distribution patterns, allowing its classification as a coastal, oceanic, southern and northern species. The community descriptors – species richness, total abundance, Pielou and Shannon index – all had patterns associated with the cross-shelf and alongshore axes. Over a temporal scale, species abundance and community descriptors showed strong seasonal patterns. Abundances decreased offshore but diversity increased towards the oceanic area, revealing an inverse relationship between total abundance and the Shannon index. Diversity was positively related to sea surface temperature. Although the most abundant species were concentrated within the coastal zone, euphausiid abundance and diversity in the oceanic region were high. Our findings suggest that the high productivity of the upwelling region may sustain the euphausiid populations in the coastal transition zone and in the offshore region, and that high abundance and diversity across the entire region may be maintained by a highly heterogeneous environment promoted by upwelling, giving rise to an increased number of potential ecological niches.

Euphausiid community structure and population structure of Euphausia superba off Adélie Land in the Southern Ocean during austral summer 2003, 2005 and 2008

The distribution and community structure of euphausiids and the population structure of Euphausia superba were investigated mainly along 140°E off Adélie Land in the Indian Ocean sector of the Southern Ocean during the austral summers of 2003, 2005 and 2008. Euphausiids were collected from six discrete depth layers, primarily between 0 and 2000 m, using an RMT 8 net. Euphausia frigida and Euphausia triacantha mainly occurred north of the Southern Boundary of the Antarctic Circumpolar Current (SB) whereas E. superba was distributed south of the SB, and occurred abundantly in the continental slope area where Antarctic Winter Water was observed. E. frigida and E. triacantha occurred in the upper 200 m during the night whereas they were mainly found below 200 m during the daytime. Cluster analysis on stations suggested that the SB is an important biological boundary for euphausiid communities. The population structure of E. superba in 2003 was different from that of 2005 and 2008. While large mature individuals dominated in 2003, small immature krill (juveniles and subadult males) were more abundant in 2005 and 2008. Further sea-ice extension in the preceding winter in 2005 and 2008 likely provided favourable conditions for spawning and survival. A cluster analysis based on similarity of the maturity stages of E. superba revealed that mature males and gravid females (stage IIIC–E) were mainly distributed in the offshore area and mesopelagic zone, while juveniles and subadult males were found in the epipelagic zone of the continental slope area. Therefore, it is considered that E. superba migrates to the offshore area and mesopelagic zone as it grows.

The zooplankton of Marguerite Bay, western Antarctic Peninsula—Part II: Vertical distributions and habitat partitioning

The vertical distribution patterns of the dominant zooplankton in the vicinity of Marguerite Bay on the west side of the Antarctic Peninsula were studied during austral fall of 2001 and 2002, using net and concurrent environmental data. Vertical distributions of zooplankton usually were similar to those reported for other Antarctic regions. Maximum abundances of the copepods Ctenocalanus spp. and Calanus propinquus, the euphausiids Euphausia superba, Euphausia crystallorophias, and Thysanoessa macrura, and appendicularians primarily occurred in shallow Antarctic Surface Water (<100 m) or the upper pycnocline. The copepod, Oncaea spp., mysids, and ostracods had the deepest distributions (>250 m), in warmer modified Circumpolar Deep Water. Other dominant copepods ( Calanoides acutus, Metridia gerlachei, Oithona spp., Paraeuchaeta spp., and Rhincalanus gigas), pteropods, and chaetognaths had depths of maximum abundance within the pycnocline or in deeper warmer waters. Overlapping depth distributions suggest that E. superba would have the highest prey encounter rates with M. gerlachei, Ctenocalanus spp., C. propinquus, and Oithona spp. during fall, although most of the copepod community was deeper than the euphausiid community. Even though the three euphausiid species occupied similar depth ranges on average, at any given location E. superba, E. crystallorophias, and T. macrura depths of maximum abundance often did not overlap, suggesting vertical habitat partitioning behavior. The vertical patterns of copepods, euphausiids, amphipods, and mysids did not have a consistent association with the distributions of pigments, temperature, salinity, or density. Instead, the observed vertical distributions are mainly attributed to different behaviors, including seasonal vertical migration to deeper water for overwintering (i.e., C. acutus, R. gigas, ostracods, chaetognaths, pteropods) and vertical habitat partitioning to reduce competition (i.e., euphausiids). Migration into deep water and aggregation behavior (i.e., euphausiids) also reduce the risk of predation.

Entrainment of Antarctic euphausiids across the Antarctic Polar Front by a cold eddy

An unusual region of high meso-scale turbulence has been identified in the Indian Ocean sector of the Southern Ocean. It has been shown that this is the result of eddy shedding from the Antarctic Polar Front. These eddies may dramatically affect the local distribution of marine organisms. To investigate this, the euphausiid community structure and species composition in the region of a cold eddy within the Antarctic Polar Frontal Zone (APFZ) was investigated during April 2005. Water masses within the core of the eddy were typically Antarctic, showing they had come from south of the Antarctic Polar Front. Results of numerical analyses indicate that the euphausiid community within the survey area consisted of three distinct groups: those in APFZ waters, those at the edge of the eddy and those in the core of the eddy. These results indicate that eddies generated by the interaction of the Antarctic Circumpolar Current with the South-West Indian Ridge play an important role in transporting Antarctic euphausiid species equatorward, thus contributing to the spatial heterogeneity of the zooplankton community within the region.

Euphausiid population structure and grazing in the Antarctic Polar Frontal Zone — austral autumn 2004

The euphausiid community structure and grazing dynamics were investigated in the West Indian sector of the Polar Frontal Zone during the austral autumn 2004. Subsurface (200m) temperature profiles indicated that an intense frontal feature, formed by the convergence of the Subantarctic Front and the Antarctic Polar Front bisected the survey area into two distinct zones, the Subantarctic Zone (SAZ) and the Antarctic Zone (AAZ). Total integrated chlorophyll a (Chl a) biomass was typical for the region (<25mg Chl a m−2), and was dominated by picophytoplankton. Total euphausiid abundance and biomass ranged from 0.1 m−3 to 3.1 m−3 and from 0.1mg dry weight m−3 to 8.1mg dry weight m−3 respectively, and did not differ significantly between the stations occupied in the SAZ and AAZ (p > 0.05). A multivariate analysis identified two interacting mechanisms controlling the distribution patterns, abundance and biomass of the various euphausiid species, namely (1) diel changes in abundance and biomass, and (2) restricted distribution patterns associated with the different water masses. Ingestion rates were determined for five euphausiid species. Euphausia triacantha had the highest daily ingestion rate, ranging from 1 226.1ng pigment (pigm) ind−1 day−1 to 6 029.1ng pigm ind−1 day−1, whereas the lowest daily ingestion rates were observed in the juvenile Thysanoessa species (6.4–943.0ng pigm ind−1 day−1). The total grazing impact of selected euphausiids ranged from <0.1μg pigm m−2 day−1 to 20.1μg pigm m−2 day−1, corresponding to <0.15% of the areal Chl a biomass. The daily ration estimates of autotrophic carbon for the euphausiids suggest that phytoplankton represent a minor component in their diets, with only the sub-adult E. vallentini consuming sufficient phytoplankton to meet their daily carbon requirements.

Species composition and spatial distribution of euphausiids of the yellow sea and relationships with environmental factors

We investigated species composition and spatial distribution of the euphausiid community in the Yellow Sea and identified the relationship with environmental factors (temperature, salinity, chlorophyll a, nitrate, phosphate, and silicate) using bimonthly data from June, 1997 to April, 1998. The environment varied during the sampling period. In warm seasons, thermocline was well developed rendering lower temperature and higher salinity and nutrient concentrations in the bottom layer. During cold seasons the water column was well mixed and no such vertical stratification was noted. Horizontal distribution of temperature, however, differed slightly between near-coast and offshore areas because of the shallow depth of the Yellow Sea, and between southern and northern areas because of the intrusion of water masses such as Yellow Sea Warm Current and Changjiang River Diluted Water. Four euphausiid species were identified:Euphausia pacifica, E. sanzoi, Pseudeuphausia sp. andStylocheron affine. E. sanzoi andS. affine were collected, just one juvenile each, from the southern area in June and December, respectively.Pseudeuphausia sp. were collected in the eastern area all the year round except June.E. pacifica occurred at the whole study area and were the predominant species, representing at least 97.6% of the euphausiid abundance. Further, the distribution pattern of the species was varied in regards to developmental stages (adult, furcilia, calyptopis, egg). From spring to fall,E. pacifica adults were abundant in the central area where the Yellow Sea Bottom Cold Water prevailed. Furcilia and calyptopis extended their distribution into nearly all the study area during the same period. From late fall to winter, adults were found at the near-coastal area with similar pattern for furcilia and calyptopis. The distribution pattern ofE. pacifica was consistent regarding temperature, salinity, and three nutrients during the sampling period, whereas chlorophyll a showed a different pattern according to the developmental stages. The nutrients should indirectly affect via chlorophyll a and phytoplankton concentration. With respect to these results, we presented a scenario about how the environmental factors along with the water current affect the distribution ofE. pacifica in the Yellow Sea.

Cross-shelf life-stage segregation and community structure of the euphausiids off central Oregon (1970–1972)

Abstract Community structure and zonal distribution of euphausiids along the Newport Hydrographic line (44°39.1′N) off central Oregon, USA, were examined from bi-weekly oceanographic surveys ( n = 48 ) carried out from January 1970 through July 1972. We explored the associations among the euphausiid community composition (14 species) and changes in sea-surface temperature (SST), sea-surface salinity (SSS), coastal upwelling index (CUI), sigma-t, day vs. night sampling time, and distance from the coast. This period included the 1970–1971 La Nina and, at low latitudes, the 1972 El Nino events; this study shows that this El Nino event did not reach the Oregon coast as have most other most recent events. Multivariate analyses were done to identify the cross-shelf environmental gradients that most influence temporal changes in the euphausiid community structure. Non-metric multidimensional scaling (NMDS) indicated that the euphausiid community separates into coastal and oceanic assemblages, with a species-richness gradient located about 45 km from shore, over the shelf-break. In the inshore zone, positive CUI (upwelling) was associated with Thysanoessa spinifera, while negative CUI (downwelling) was associated with the presence of the oceanic species assemblage during October–November. Indicator species analysis (ISA) showed that two species were good indicators for the oceanic environment: Thysanoessa longipes and Nematoscelis atlantica. Three species (Thysanoessa gregaria, Nematoscelis difficilis, and Tessarabrachion oculatum) were moderate indicators for the oceanic environment because, during fall and winter downwelling events, they may be transported over the continental shelf. High densities of larvae and juveniles of T. spinifera were found nearshore ( 18–108 km from the coast). Euphausia pacifica was relatively homogeneously distributed in shelf and offshore waters, but its larvae were recorded chiefly inshore in mid-summer and offshore during the rest of the year. Our data showed a strong cross-shelf gradient in euphausiid assemblages and life-stage-segregated distributions for T. spinifera and E. pacifica, suggesting active population maintenance strategies within this upwelling region.

Mesozooplankton interactions with the shelf around the sub-Antarctic Prince Edward Islands archipelago

Mesozooplankton surveys were conducted in April/May for four consecutive years (1996–1999) in the vicinity of the Prince Edward Islands (PEIs), Southern Ocean. The PEIs are located in the Polar Frontal Zone, directly in the path of the east-flowing Antarctic Circumpolar Current. Zooplankton were collected by oblique tows using a Bongo net fitted with 300 μm mesh. The abundance, biomass and average size of the mesozooplankton in the upstream (USR), inter-island (IIR) and downstream (DSR) regions indicated that some groups and species were significantly affected by their interaction with the shallow shelf waters of the PEIs. Total mesozooplankton abundance and biomass were typically highest in the DSR, but no consistent pattern was evident in the USR and IIR. Copepods, euphausiids and fish were generally of a low average size in the IIR. This small size was largely attributed to the reduced abundance, or complete absence, of mesopelagic species from the shelf region. Of total biomass, the mesopelagic species Euphausia longirostris, Euphausia similis, Pleuromamma abdominalis, Paraeuchaeta biloba and Oncaea antarctica together contributed an average of 16% to the USR, 2% to the IIR and 15% to the DSR. Conversely, epipelagic species showed no consistent pattern of abundance and biomass distribution between regions. The low incidence of mesopelagic species over the island shelf was attributed mainly to reduced advection of deep water into the shelf region (average depth = 200 m), rather than predation, particularly during the through-flow mode between the islands. This resulted in substantial regional differences in euphausiid community structure. The epipelagic species Euphausia vallentini and Thysanoessa vicina completely dominated the IIR, comprising on average 89% of total euphausiid biomass in this region. However, predation may be important during the water-trapping mode between the islands. Advection of zooplankton into the IIR appeared to be affected by the proximity of the Subantarctic Front (SAF). In 1996, when the SAF was far north of the PEIs, reduced current velocities resulted in some degree of water retention over the shelf and an increased predation impact. Conversely, when the SAF was close to the PEIs in 1999, more large plankton were transported over the island shelf. High current velocities and productivity associated with the SAF appear to increase the biomass and size of allochthonous zooplankton/nekton advected into the IIR, and consequently may have increased the availability of prey to land-based predators. The long-term southward movement of the SAF recently observed in the vicinity of the PEIs may therefore have important implications for the ecosystem of these islands.

Distribution of euphausiid assemblages in the Mediterranean Sea

Eleven species of euphausiids from 24 Isaac-Kidd Midwater Trawl (IKMT) night collections taken at stations throughout the Mediterranean Sea were counted. The frequency of occurrence and dominance of individual species and percent similarity faunal analysis of the euphausiid community were used to describe changes in faunal composition between geographical areas and differences in vertical distribution. Although most species were widespread, three distinct patterns of abundance were apparent: Euphausia krohnii, Nematoscelis megalops, Meganyctiphanes norvegica, and Stylocheiron abbreviatum predominated in western basin areas west of the Tyrrhenian Sea; Euphausia hemigibba, Thysanopoda aequalis, and Stylocheiron longicorne predominated in the Tyrrhenian Sea and eastern Mediterranean Sea; Euphausia brevis and Stylocheiron suhmii predominated in the eastern Mediterranean Sea. Percent similarity analysis of data from the IKMT samples and data from Ruud (1936) indicates the Tyrrhenian Sea fauna at the time of the collections was more similar to eastern Mediterranean areas than to most other areas in the western basin, although the degree of similarity was dependent, to some extent, on the depth at which the samples were collected. The composition of euphausiids living above 150 m at night in this area was more similar to eastern basin areas, while the composition of deeper living forms was more similar to those of the rest of the western basin. Comparison of euphausiids collected at three points over a 60 year time-span in the Balearic Sea shows the similarity in composition to be greater within the area over time than between adjacent areas in the western Mediterranean Sea.